Embodiments described herein relate generally to the placement of electrical components on a printed circuit board, and more particularly, subsurface mounting of discrete electrical components on internal printed circuit boards layers.
Some known electronic devices manufactured for high-speed signal transfer (e.g., greater than 10 Gigabits per second Gb/s) use printed circuit boards (PCBs) having dielectric materials with specific characteristics. Often, such dielectric materials are configured to minimize losses, thereby facilitating the high-speed signal transfer. Such known dielectric materials are typically expensive, at times being five to seven times the cost of a standard, commonly used PCB substrate (e.g., FR-4). As such, the efficient use of space on the PCBs is desirable.
Often, such PCBs have differential pairs and/or impedance-controlled strips (e.g., a transmission line such as a microstrip and/or stripline) to minimize impedance mismatches that can result in signal degradation. In some known high-speed applications, it may be desirable to use, for example, serial capacitors (i.e., alternating current (AC) capacitors) to provide AC isolation between a transmitter component and a receiver component on a PCB. Such capacitors, however, can introduce a characteristic impedance mismatch into the circuit. As such, the PCBs often employ signal vias to provide connectivity to internal layers of the PCB, which generally are back drilled to reduce the via stub. Moreover, the signal vias can degrade signal quality by increasing loss and/or cross-talk, as well as having parasitic resonances. In an effort to minimize these effects, the PCBs often include several ground vias to provide return signal paths, improve characteristic impedance mismatches, and reduce or eliminate signal-via-to-signal-via cross-talk. The inclusion of several ground vias, however, reduces the usable space of the PCB (e.g., portions of signal layers below the AC capacitors often become unusable) and can often result in challenges with routing signal traces.
Thus, a need exists for improved apparatus and methods for placement of discrete electrical components on internal layers of a PCB to increase component density of the PCB.